A malathion-specific esterase in a highly resistant strain of the red flour beetle Tribolium castaneum Herbst

A resistant strain of Tribolium castaneum was kept under repeated selection pressure with malathion to give high levels of resistance. General esterase, malathion-specific carboxylesterase, glutathione S-transferase and target site sensitivity were compared in this and a susceptible strain. Resistance in the highly resistant strain (resistance ratio 2812) was highly specific for malathion and was greatly suppressed by non-toxic carboxylesterase inhibitors. Acetylcholinesterase and esterase activity (alpha-naphthyl acetate, beta-naphtyl acetate or alpha-naphthyl butyrate substrates) was significantly lower in the resistant strain. However, the rates of both in vitro and in vivo malathion hydrolysis' using [C-14-methoxy] malathion, were many times greater in the resistant strain. in vitro inhibition studies suggested that resistance was due to a carboxylesterase unique to the resistant strain and not to acetylcholinesterase, general esterase or glutathione S-transferase mechanisms.